A little over a year ago I had to change rear brake rotors. Got new ones and installed with new pads, calipers were OK. During assembly I bent the rotor shield while setting the caliper. A I turned the rotor, I noticed that the rotor seemed out of round, had a high spot on the outside edge of the rotor.

I have a rotary micrometer so I checked the rotor, sure enough, the rotor was not concentric with the hub. There was a 0.050"-0.060" run out. That was a lot, WAY MORE than I liked but as I already had it back together. I just marked the axle face where the high spot was figuring if it was too bad, I could throw some weights opposite the high spot to mitigate any out of balance issues.

Here is a video, big file.

rotor run out

If any of you have taken the live axles apart, you know what a pain it is to do it. There is also a high risk of damaging the inner rear bearing seal, it is a rotary seal and can be damaged easily. Well, sure enough there was a vibration, there had always been a slight vibration but now it was much worse.

I recalled when I bought the rotors, the counter guy asked me if the rotors were for the drive axle or steer axle, for the drive axle they were about $70.00 each, for the steer axle about $140.00 each. I was told better machining but shouldn't be a problem-----mistake #1.

I checked the passenger's side and it was OK, less than o.oo3" out of round.

Last week I dissembled the left axle AGAIN, Took the rotor off after recording the run out, back to the supplier. They put the rotor on their lathe and claimed that the brake surface was true and had less than 0.001 run out! I told them that yes I had measured that BUT it was the centering of the rotor on the hub and eccentricity was way out. They could not believe it was that bad.

They actually gave me full credit for the rotor and I got a steer axle "premium" rotor. On their lathe this one was dead on and concentric with the center hole.

Here is where it gets interesting!! The brake rotor Is located on the hub by 6 bolts ONLY! there are no locating pins or shoulders to insure the rotor is concentric with the hub!Inserting the 6 bolts finger tight, I could move the rotor 0.012 side to side, That sure is enough out to get vibration!

I machined an aluminum disk so that I could hold the rotor concentric with the hub while tightening the 6 retaining bolts. Wasn't quite as simple as that. the center hole of the rotor was 0.080" smaller than the hub bearing/seal surface. I cut a removable 0.080" shim to match the hub diameter to the rotor diameter. Once the retaining bolts were secure, I could remove the disk and then fish out the shim.

centering disk:

Disk in the hub:

Rotor centered with the disk


The rotor now has a run out of less than 0.005", vibration? Haven't had a chance to drive it yet but hopefully this week some time.

Part numbers below are for the Eaton 15040S Axle w/6 bolt wheel lug bolts. It is a problem to find the locking washer for the wheel bearing retaining nuts. I don't have a part number and could not match it at my local truck parts store.


Brake rotor part number 14.75":
Drive axle--Cen 121.82001
Steer axle--Cen 120.82001 (Premium rotor)
Brake pads: Raybestos ATD224M

Thanks Ed. Nice write up for our archives. How did you know it was the rotors in the first place? I would have had the tires balanced first. We all appreciate your mechanical knowledge here.

I had replaced the tires and all 4 were balanced. I also had dynamic balancers later and that wasn't acceptable. The vibration was there for quite awhile. Then I noticed the brake rotors both rears had cracked. Replaced the rear rotors and then the vibration was much worse. Took the wheels off and miked the rotors. The passenger's side was OK but the driver's side was WAY OUT. You could actually see it as the rotor was turned. 0.050"-0.060" is what I measured. I could not change the rotor at that time so I lived with it for 6 months.

The main point here is that the rears on the Spartan chassis need to be well balanced or you will get vibration. Having Bilstein shocks makes even worse as they are heavily valved and will transmit a higher level of NVH back into the chassis.

YES, I tried it, I changed the rear shocks to Monroe and vibration was reduced at lower speeds (<55 MPH). At 65-75 about the same with either shock.

I am NOT crazy about the mounting of the rear rotors to the hub. Unless you would get lucky and have near zero run out after mounting, I believe vibration would be there. Dynamic balancers should work but they have their limits.

Why don't they chuck the rotor up and cut the diameter concentric to the hub mount?

quote:

Originally posted by MWrench:
Dynamic balancers should work but they have their limits.

Good point, now we know Dynamic balancers cannot make up for oval rotors. I once had oval rims on an old dump truck. I found out you cannot balance oval. I had to replace the rims with new ones.

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Good quality is hard to find now a days.

quote:

Originally posted by Duane88:
Why don't they chuck the rotor up and cut the diameter concentric to the hub mount?

They could but They would have to remove the rotor to install the new seal and that would take away any accuracy they machined.

Stupid design!! As long as there is NO LOCATING flange, pins, removing the rotor and centering again will be guess! Sure on trucks? Low speed? No problem! Done it many times!

Now it is for my comfort.

Ed you are not giving me the warm fuzzies. You know I don't have as nearly as many miles--Yet--- You know your and my drivetrain are very much the same. Same rear end so I am assuming my rotor set up is the same.

You mechanical aptitude far outshines mine. I have always considered myself a rough mechanic, rough carpenter. I attribute to my poor hand eye coordination Any wonder I got cut from the basketball team for going deer hunting??

I'm thinking I wouldn't be able to afford to pay someone to fix what you are able to do yourself....

Guess I'm just bummed Steve VW got to go south and I had to use a towrope to pull my trailer out from underneath the snow and out to the driveway. It was between DBarth and the shed so nowhere to blow the snow. minus 4 this morning. I needed the trailer to do a salvage that wouldn't wait for the thaw

Look forward to seeing you this June. Do you want me to save some snow for you? I can put some in the freezer.

I think it should go without saying "I always worry when I get vibration from my rear!"

My Glassnose of course has drum brakes in the back , I feel left out I may not ever get to experience that vibration in my rear!!

On one of the TV car shows, if memory serves me, I saw a sexy lil machine that spins the rotor on the axel and machines the surfaces, wonder if that would also do the periphery.

An after thought, how does it work with posi-traction? (sp)

I had one of those machines (oddly enough, bought cheap at the pawn shop next to my office) as my '87 Honda Accord loved to warp rotors. It worked reasonably well (although a pure PITA to set up). I sold it when I discovered the vendor no longer sold 12-packs of assorted grits of abrasive pads - minimum order, 250, one grit grade only.

I also had one of those on vehicle brake lathe if you can call it that. I also had an on vehicle wheel balancer. Had a motor/drum to spin the wheel assembly and an attachment that mounted to the wheel. That had a series of rotary controls that one could add/subtract weight and move that weight around the wheel to find the smoothest running spot. The dials on the controls would tell you how much weight to add and where to add it.

Spinning up dullies on the vehicle would be a bit of a problem!

GOOD NEWS! Test drive yesterday from home to Half Moon Bay was one of the smoothest drives I have had in a long time. no detectable wheel vibration all the way to 80 MPH. really nice running at 70 MPH.

It is always great to hear of a win!!!